This seems like a classic case of overheating. The computer is slow because the CPU is throttleing back to prevent heat damage and you see artifacts because of the GPU overheating as well. You must improve the overall cooling and airflow of your case. I would remove both the CPU and GPU coolers and after cleaning the cores, reapplying a good quality TIM. Make sure you have at least one fan sucking in cold air and one blowing out hot air from the case. Once you improve the cooling, you should see a big difference overall in your computing experience...